1. Field of the Invention
The present invention relates to the field of Doppler ultrasound imaging in living tissue. Specifically, this invention relates to an apparatus for processing ultrasonic data suitable for displaying upon a suitable medium, such as a video display screen, for observation and diagnosis by medical personnel.
2. Prior Art
Images of living organisms typically utilize methods that pass various types of radiation through the body of the animal and measure the output with a suitable detector. For instance, x-ray images are generated by producing x-rays external to the body, passing the x-radiation through the body and observing shadows produced on x-ray sensitive film. Ultrasonic images, in contrast, are formed by producing ultrasonic waves using a transducer, passing those waves through the body, and measuring the properties of the scattered echoes from reflections inside the body using a receptor. Ultrasonic imaging apparatus may be distinguished from other medical imaging apparatus in the respect that they allow the display of soft tissues within the body which show various structural details such as organs and blood flow.
An ultrasonic imaging apparatus utilizes a probe which contains elements for transmitting acoustic pulses throughout tissue, which probe typically also contains receiving circuitry which allows reception of the reflected acoustic pulses. Such probes typically comprise a plurality of elements in a linear fashion such that each of the elements are fired at time intervals to focus on specific parts of the body. In other systems, multiple elements are simulated by means of a moveable mechanical element within the probe wherein the acoustic pulses are transmitted at various intervals along an axis. Each reflective pulse from the acoustic pulses emitted may then be received by a receiving unit located in the probe and transmitted to circuitry within the ultrasound apparatus for processing and generation of a display. This display, known as a b-mode image or two-dimensional image of blood flow velocity, may then be generated by the apparatus and displayed on a video monitor for diagnosis and examination by an attending operator or physician.
The basic principle used in applying the Doppler method for ultrasonic imaging in a pulsed Doppler ultrasound apparatus is described as follows. When blood flow within a living subject is subjected to ultrasonic waves, corpuscles are caused to vibrate slightly while moving and reflect those ultrasonic waves. Because of the corpuscle velocity, the frequency of the reflected waves changes from that of the transmitted waves because of the Doppler effect. The frequency shift may be detected and the amount of the shift may be displayed on a video screen for imaging blood flow in the living subject. Since the amount of shift of the transmitted waves is in relation to the blood flow velocity, the amount of blood flow and the speed of the blood flow may be observed. Noise and other signals (clutter) which have Doppler shift but don't represent blood movement in the body are filtered out, so that the image produced only represents blood flow in motion. In color Doppler imaging the frequency information is then used as blood flow information for forming a two-dimensional image or profile of the blood flow velocity.
One such apparatus used in displaying information obtained from ultrasonic pulses transmitted in the human body is shown in FIG. 1 as imaging system 100. Imaging system 100 generally comprises a probe 101 which is coupled via line 110 to transmitter/receiver circuitry 102. Transmitter/receiver circuitry 102 is designed so that the elements in probe 101 will be fired at specified time intervals, with reflective pulses being detected using probe 101 at another given time interval. Transmitter/receiver circuitry 102 is coupled to a control unit 109 via bus 120. Control unit 109 controls all circuitry in the imaging system via bus 120. Control unit 109 is further coupled to a keyboard 125 and a mouse, trackball or other device 126 for movement and control of information shown on video display 130.
Once a pulse is received by the receiver circuitry within transmitter/receiver 102, such information is transmitted by line 111 to RF (radio frequency) processor 103 for further processing. RF processor 103 processes the RF information to produce an envelope signal and in-phase (I) and quadrature (Q) Doppler signals. These signals are further transmitted via line 114 to a scan converter 105 and to a Doppler processor 106 via lines 114 and 113 for generation of black and white ultrasound information on video display 130. Information generated by Doppler processor 106 via I and Q signals output from RF processor 103 are transmitted via line 115 to scan converter 105. Scan converter 105 then integrates information received from RF processor 103 and Doppler processor 106 and transmits scan line information to video processor 127 via line 116. In addition to information passed to scan converter 105 and Doppler processor 106, RF processor 103 transmits I and Q signals via line 112 to color flow processor 104. Color flow processor 104 is also controlled by control unit 109 via bus 120. Color flow processor 104 is used for detecting Doppler shift and blood flow information in living tissue, and thus transmits such information via line 117 to a color scan converter 108. Such color information is used to graphically represent on video display 130 moving blood flow in a living organism. Color scan converter 108 is used to interpolate scan line information obtained from color flow processor 104, and transmit that information on line 118 and thus to video processor 127 for representation of blood flow in the human body. Video processor 127 then utilizes information obtained from scan converter 105 for display of black and white ultrasound information and color information obtained from color scan converter 108 to generate a color image showing blood flow overlaid on a black and white image showing stationary tissue suitable for output on a video display such as 130 via line 119. Such information may be transmitted in National Television Standards Committee (NTSC) format and thus be stored on video tape for later clinical examination by attending medical personnel.
A more detailed representation of a prior art color flow processor 104 shown in FIG. 1 is shown in FIG. 2. 104 shown in FIG. 2 is representative of a prior art color flow processor. As is shown in FIG. 2, line 112 contains I and Q data from RF processor 103 shown in FIG. 1 to two analog filters 201. The analog filters 201 are used by the color flow processor 104 to filter out clutter information contained within the Doppler signal. In other words, analog filters 201 will eliminate signals contained within the analog Doppler data that indicate no motion. Typically, in a prior art system such as color flow processor 104 shown in FIG. 2, objects with little or no motion generate reflected signals below 150 Hz which will be filtered out by analog filters 201. Analog filters 201 are regulated by color sample volume (CSV) controller 200, and sample each frequency a fixed number of times to generate a continuous average for each color sample volume (CSV). The CSV controller 200 thereby commands analog filters 201 to sample the Doppler signal at fixed time intervals only.
Coupled to analog filters 201 via lines 210 are matched filter analog-to-digital (A/D) converters 202 shown in FIG. 2. A prior art system, such as color flow processor 104, typically comprises a plurality of A/D converters coupled to filters such as analog filters 201, which then digitize the analog signal after clutter has been removed. The matched filter A/D converters 202 are 12 bit resolution converters which take the signal generated by the analog filters 201 and digitize it to 12 bits of accuracy. This information can then be transmitted to a digital filter 203 shown in FIG. 2 via lines 211. Digital filter 203 allow additional removal of clutter information from the Doppler signal which has been digitized, so that an uncorrupted Doppler signal may be obtained which is useful for display upon video display 130 in ultrasound imaging system 100. After the additional clutter has been removed by digital filter 203, the processed information is transmitted via lines 212 to parameter estimator 204. Parameter estimator 204 is used for generating samples of each point along vectors for display on video display 130. Parameter estimator 204 determines the phase shift and direction of that shift for each signal in each CSV to generate flow information. This flow information generally comprises velocity and amplitude information for reflected Doppler signals. In addition to amplitude and velocity information parameter estimator 204 generates variance information. Variance is generally the difference between maximum frequencies and minimum frequencies for a particular time interval. Parameter estimator 204 is further coupled to interface 205 via bus 214 and output line 213. Interface 205 controls signals output from parameter estimator 204 into interface 205 via line 213. Interface 205 then generates appropriate information over line 117 for transmission to color scan converter 108 shown in FIG. 1.
Essentially, color flow processor 104 shown in FIG. 2 requires extensive analog filtering via analog filters 201 prior to digitization of the input Doppler signals for color flow parameter extraction. This is because there is too much clutter in the signal prior to digitizing to resolve a signal with only 12 bits of information. Since various analog processing must be performed prior to digitization, flexibility is lost since information which is filtered out might otherwise be available by adjusting filter parameters. Therefore, an apparatus is required for determining Doppler flow data which allows convenient adjustment of filters for ultrasonic imaging. In addition, since the prior art limited digitization of flow data from color scan vectors to 12 bits, an increase in resolution of such digitized signals would be useful.